The high electrical-conductivity layer of the upper mantle beneath the sea west off north-west Kyushu is strongly indicated by the GDS (Geomagnetic Depth sounding) modeling, inferred from the observed data at the sites densely distributed in central Kyushu. The aim of this study is to make clear the conductivity structure beneath this area. The results obtained are as follows:(1) The preliminary result obtained by the sea bottom observation with OBEM in Danjyo basin and the MT observations in land is that the upper mantle is possibly conductive beneath the area between Danjyo Islands and Shimabara Peninsula. This is consistent with the model obtained by the GDS in central Kyushu.(2) The MT observations also made in Tsushima Islands to make clear the conductivity structure, with special attention to the northern extension of the conductive mantle beneath Danjyo basin. The preliminary result indicates the upper mantle is resistive beneath Tsushima Islands.(3) A new type OBEM that can measure the geomagnetic variations in the flow should be developed to observed successfully in this shallow sea.